A silicon nitride thin film has been used for various purposes in manufacturing a semiconductor device. In recent years, with continuously miniaturizing semiconductor devices, a silicon nitride thin film of 30 nm or less, or even 10 nm or less has been needed. By way of example, Korean Patent Laid-open Publication No. 10-2011-0102686 (International Patent Application Publication No. WO2010/025024) discloses that a dielectric material such as silicon nitride is preferable as a liner material of an isolation trench and that a thickness thereof is from 20 Å to 100 Å, i.e., 2 nm to 10 nm. Further, U.S. Patent Laid-open Publication No. 2012/0085733 discloses that when an uneven surface is covered with a spacer layer having a uniform thickness in order to increase a pattern density after a lithography process, silicon nitride can be used as a material of the spacer layer. In order to form a pattern having a half-pitch of 15 nm for manufacturing a semiconductor device, a nitride spacer needs to be formed to have a thickness of about 15 nm on the uneven surface [Chen Y, Xu P, Miao L, et al.; “Self-aligned triple patterning for continuous is scaling to half-pitch 15 nm” SPIE Advanced Lithography. 0001; 79731P-8. doi: 10.1117/12.881645].
U.S. Pat. No. 8,143,131 discloses a method in which a silicon oxide spacer is formed on a gate stack of a transistor and then a silicon nitride spacer is formed to have a thickness of 20 Å to 200 Å, i.e., 2 nm to 20 nm, on the silicon oxide spacer. The atomic layer deposition method in which gaseous materials for forming films are alternately brought into contact with a substrate surface is particularly advantageous in forming a film having a uniform thickness on a surface including a trench having a small width and a high aspect ratio. A thickness of the spacer formed on the gate stack needs to be precisely controlled. Instead of the conventional low pressure chemical vapor deposition method, the atomic layer deposition method for forming a silicon nitride thin film at a low temperature has been actively studied [Raaijmakers I; “Current and Future Applications of ALD in Micro-electronics” ECS Transactions, Volume 41, Issue 2, pp 3-17 (2011). doi: 10.1149/1.3633649].
However, in spite of such a need, there has not been known a method for forming a silicon nitride thin film having excellent properties to a uniform thickness on a surface including a trench having a small width and a high aspect ratio by the atomic layer deposition (ALD) method at a temperature of 560° C. or less, 520° C. or less, 500° C. or less, or 450° C. or less.